This invention relates generally to plastic injection molding. More specifically, the present invention relates to a pressurization control unit for gas assisted injection molding.
The invention is particularly applicable to a control unit which is utilized during the injection of a viscous fluid, such as a molten plastic, and a non-viscous fluid, such as a gas, into an injection mold in a process known as gas augmented injection molding of plastic materials. However, it should be appreciated by those skilled in the art that the invention has broader applications and may also be adapted for use in many other injection molding environments for both a relatively viscous fluid, such as a plastic or wax, and a relatively non-viscous fluid, such as a gas or liquid, are injected into a mold cavity.
Injection molding processes have been widely used not only for the production of molded articles made of various thermoplastic resins but also for the production of lost wax masters used in the investment casting process. The conventional solid injection molding process generally uses a thermoplastic material.
Solid injection molding employs the steps of injecting a plasticized (melted) thermoplastic material under high pressure into a finite mold space and then allowing the material to cool sufficiently so that it rehardens to the extent that it can retain its shape after removal from the mold. Thermoplastic materials generally shrink during rehardening and unfortunately this shrinkage is exaggerated in heavier wall sections, bosses, ribs, gussets, etc. This usually results in sink marks and warpage in the molded products. As a remedy for this, proposals have recently been made to fill the mold cavity with a plasticized thermoplastic material to a volume less than 100% of the mold space and to utilize an inert gas injected under pressure into the partially plasticized material, as it is cooling and rehardening, to fill the rest of the volume of the mold cavity. The gas enters the part and moves along the paths of least resistance therein. Such paths are normally those areas where the thermoplastic body is thicker and has slower cooling sections such as ribs, flow channels, chamfers, etc. In this way, with a suitably designed part, a continuous network of hollowed out sections can be provided. This network of gas channels provides a uniform pressure distribution system throughout the mold space during part rehardening and cool down, thus minimizing internal stresses. The gas pressure is advantageous for holding the plastic material up against the mold surfaces during rehardening so that sink does not take place at the mold surfaces.
While several types of such nozzles are known to the art, there is no suitable control mechanism thus far available at this time for controlling the actuation of the gas flow into and out of the mold cavity during the process of gas assisted injection molding.
Accordingly, it has been considered desirable to develop a new and improved injection molding apparatus which would overcome the foregoing difficulties and others while providing better and more advantageous overall results.